EP 1 363 062 A1 discloses a line pipe for the transport of deep-frozen media, which consists of at least two metal pipes arranged concentrically and at a distance from one another, an insulating layer being arranged in the annular gap between the metal pipes, and the annular gap being evacuated. According to an advantageous embodiment, the metal pipes are longitudinally welded and corrugated high-grade steel pipes. On account of the corrugation, line pipes of the type described can be produced in great lengths. The insulating layer consists of plies arranged alternately one above the other and composed of insulating material and of a material having a reflecting surface. Glass fibre fleece and aluminium foils are preferred. An insulation composed of these materials in combination with a high vacuum is designated in cryotechnics as superinsulation. The known line pipes are used preferably as transfer lines for condensed gases, but also as cryostats for superconductor cables.
The known line pipes are distinguished by high economic efficiency, since, because of the great length which can be produced, a multiplicity of junction connections are unnecessary, as compared with lines consisting of smooth pipes. Owing to the flexibility, the lines can also be laid in narrow radii.
One disadvantage of the known lines is that, because of the great length, evacuation from the ends is highly time-consuming.
EP 1 363 062 therefore proposes to separate the exterior of the metal pipes at predetermined intervals and to weld, vacuum-tight, a pumping connection piece to the outer metal pipe at each orifice. For this purpose, metal-ring halves are placed onto the two ends of the outer metal pipe in the region of the orifice and are welded to the ends of the outer metal pipe. Half-shells consisting of a smooth-walled metal pipe are then placed onto the metal-ring halves and welded to the metal-ring halves and in their longitudinal directions. One of the half-shells has a connection piece for a vacuum pump. This design has the disadvantage that the metal-ring halves and the connection piece project out of the circumferential surface of the outer metal pipe. It is consequently not possible to draw the line pipe into narrow ducts.
Designs with a projecting pumping connection piece which have similar disadvantages are disclosed in DE 31 42 702 A, U.S. Pat. No. 6,257,282 B1, WO 01/27514 A1 and U.S. Pat. No. 4,046,407.